1. Field of the Invention
This invention relates to a method for the selective hydrodefluorination of an N-substituted 3,4,5,6-tetrafluorophthalimide. The product may be hydrolyzed to form 3,4,6-trifluorophthalic acid, which, in turn, is a useful chemical intermediate for the synthesis of 2,4,5-trifluorobenzoic acid. The latter is a valuable chemical intermediate for the further preparation of quinolone antibacterials.
2. Prior Art
Various methods for the preparation of 2,4,5-trifluorobenzoic acid, or precursors thereof, are disclosed in the literature. O'Reilly, N. J.; Derwin, W. S.; and Lin, H. C.; SYNLETT, June 1990; 339; disclose the preparation of 3,4,6-trichlorophthalic acid by hydrodechlorination of 3,4,5,6-tetrachlorophthalic acid with zinc dust in aqueous sodium hydroxide.
U.S. Pat. No. 4,981,999 to O'Reilly, Derwin, and Lin discloses the preparation of 3,4,6-trichlorophthalic acid by hydrodechlorination of 3,4,5,6-tetrachlorophthalic acid or anhydride with a hydrodechlorinating metal, such as zinc, in the presence of a base, such as aqueous sodium hydroxide. The trichlorophthalic acid may then be converted to the corresponding trifluorobenzoic acid. The patentees provide comparative data showing that the subject process of selective dehalogenation using zinc dust and aqueous sodium hydroxide produce 3,4,6-trihalophthalic acid, is efficient for the conversion of tetrachlorophthalic anhydride to 3,4,6-trichlorophthalic acid (97.8% yield) and substantially ineffective for the conversion of tetrafluorophthalic acid to 3,4,6-trifluorophthalic acid (about 3% yield of desired product with about 96% of hydroxy-trifluoro-phthalic and -benzoic acids).
U.S. Pat. No. 4,935,541 to Nowak and Lin discloses the preparation of trifluorophthalic acid by preparation and fluorination of an alkyl or aryl trichlorophthalimide followed by hydrolysis to the trifluorophthalic acid.
U.S. Pat. No. 4,769,493 to Ito, Matsushita, Shimizu, and Ishikawa discloses a similar process involving fluorination of an organo-N-tetrachlorophthalimide to form the corresponding N-tetrafluorophthalimide and hydrolysis to the tetrafluorophthalic acid.
U.S. Pat. No. 4,925,966 discloses the reaction of a tetrafluoroisophthalonitrile with a metal hydride, such as sodium borohydride, to give 2,4,5-trifluoroisophthalonitrile.
Treatment of tetrafluorophthalonitrile with zinc in H.sub.2 SO.sub.4 led to 3,4,6-trifluorophthalonitrile in an 88% yield (JP 01,160,944 (1988); CA 112:55243t). In a similar vein, 2,4,5,6-tetrafluoroisophthalonitrile was treated with zinc in aqueous sulfuric acid leading to 2,4,5-trifluoroisophthalonitrile (JP 01,258,639 (1989); CA 112:178350h); the same conversion was also reported using zinc in acetic acid solvent (JP 02,117,643 (1990); CA 113:131769g). Also, 3-chloro-2,4,6-trifluoroisophthalonitrile was converted to 3-chloro-2,6-difluoroisophthalonitrile using zinc in acetic acid solvent (JP 02,169,542; CA 113:190932c).
The conversion of pentafluorobenzonitrile to 2,3,5,6-tetrafluorobenzonitrile, by reaction with zinc in aqueous potassium dihydrogenphosphate is reported in Japanese Patent 02,115,156 (1990). A similar conversion using zinc in aqueous ethanol with added acetic acid is reported in Japanese Patent 01,56,656 (CA 112:7178d).